Assuming you can get to it from beneath in the attic - just find the place where you intend to locate the system, measure 1/2 way between 2 joists, and drill a small hole at that point. Then stick something through the hole so you can find it from above (like a piece of wire). Now you know where the joists are from above: 8" (or whatever distance) from the hole in both directions.

If you have a shingled roof you can lightly beat on the roof and easily find the rafters without causing damange to the shingle. We so this when attaching roof jacks on a steep roof or cutting vent holes.